To late, but I would have voted for the Tamron (non-VC). Super sharp and excellent focus in low light. I used the Sigma 24-70 f/2.8 and was VERY disappointed. It had some back focus issues (which I was able to fix with my camera body) and it was awful when trying to focus in low light conditions.
I did a wedding 2 years ago at a restaurant that had high dark ceiling, dark walls and small lights. The atmosphere was great, but the Sigma was almost useless. I had so many problems getting it to focus. Luckily I also has the Tamron 17-50 f/2.8. I put that lens on and never had a focus issue the rest of the night. It focused fast and was tack sharp. It almost never left my camera until I bought the Nikon 24-70 f/2.8.
When I bought the Tamron I also looked at the Sigma 17-50 f/2.8 and the reviews for it weren't no way near as good as for the Tamron.
Good luck with yours. (BTW I do love my Sigma 10-20mm and 30mm f/1.4)